Histology of the Central Nervous System Flashcards
cells that respond to stimuli through electrochemical mechanisms
- Excitable or inhibitory
Neurons
support cells involved in conduction speed, repair, neurotransmitter maintenance
Glia
dense network of interwoven nerve fibers &
their branches and synapses, together with glial filaments
Neuropil
Anatomy of a Neuron
- Cell body with nissl substance/bodies
- Dendrites with dendritic spines
- Long axon covered by myelin that emerges from an axon hillock
The Axon
- site where excitatory or inhibitory stimuli are initiated
Initial segment
The Axon
• distal end of the axon
- some have branching, collateral branches
Terminal arborization
The Axon
- small ends of axons
- Dilation of branch ends & contact postsynaptic cell
Telodendria
Structural Classification of Neurons
- most common, throughout CNS
- MNs, CNS interneurons
Multipolar
Structural Classification of Neurons
- sensory neurons of the retina, olfactory mucosa,
and inner ear
Bipolar
Structural Classification of Neurons
- other sensory neurons, including the dorsal root ganglia & cranial ganglia
Uni/Pseudounipolar
Structural Classification of Neurons
- neurons of the CNS lack true axons and do not
produce action potentials, but regulate local electrical
changes of adjacent neurons
Anaxonic
– Carries material from the nerve cell body to the periphery
– Utilizes kinesin, microtubule-associated motor protein that uses ATP
Anterograde transport
– Carries material from the axon terminal & dendrites to the nerve cell body
– Utilizes dynein, microtubule-associated motor protein
Retrograde transport
conveys substances from the cell body to the terminal bouton at the speed of 0.2 to 4 mm/day
- Anterograde only
- Tubulin molecules, actin molecules, & neurofilament proteins
Slow Transport
Conveys substances in both directions at a rate of 20 to 400 mm/day
- Anterograde carries membrane-limited organelles, sER components, synaptic vesicles, & mitochondria
- Retrograde carries membrane-limited organelles & molecules endocytosed at the axon terminal; path followed by toxins & viruses
Fast transport
Functional Classification of Neurons
- (afferent) receive stimuli from receptors throughout
the body
Sensory neurons
Functional Classification of Neurons
- (efferent) send impulses to effector organs
Motor neurons
Functional Classification of Neurons
- neurons under voluntary control & innervate skeletal muscle (motor neurons)
Somatic
Functional Classification of Neurons
- control the involuntary or unconscious activities of glands, cardiac muscle, & smooth muscle
Autonomic
Functional Classification of Neurons
- establish relationships among other neurons, forming complex functional circuits
- Comprise 99% of all neurons in adults
Interneurons
- Presynaptic terminal bouton contains mitochondria & synaptic vesicles → release NT via exocytosis
- 20- to 30-nm-wide intercellular space, synaptic cleft separates pre- & postsynaptic membranes
- Postsynaptic cell membrane contains:
- Receptors for the neurotransmitter
- Ion channels to initiate a new impulse
Chemical Synapse
• Permit direct, passive flow of electrical current from one neuron
to another
- Potential difference generated locally
by presynaptic action potential
• Current flow occurs via gap junctions, link pre- & postsynaptic membranes (2nm)
• Contain connexon proteins, permit
diffusion of small molecules &
electric current flow (NO vesicles)
Electrical Synapse
fibrous intercellular network of cellular processes emerging from neurons & glial cells
Neuropil
- Large number of long, branching processes
* Proximal regions are reinforced with intermediate filaments made of glial fibrillary acid protein (GFAP)
Astrocytes
• Processes can affect the formation, function, and plasticity of synapses
• Regulate extracellular ionic concentrations around neurons, with
particular importance in buffering extracellular ?levels
• Guide and physically support movements & locations of
differentiating neurons during CNS development
• Extend fibrous processes with expanded ? that cover capillary endothelial cells and modulate blood flow and help move nutrients, wastes, and other metabolites between neurons and capillaries
• Filling tissue defects after CNS injury by proliferation to form an
?
Astrocyte Functions
- K+
- perivascular feet
- astrocytic scar
• Extend many processes that wrap repeatedly around a portion of a nearby axon
- Cytoplasm gradually moves out, leaving compacted layers of cell
membrane, myelin
• Will enwrap axons from multiple neurons
• Usually appear as small cells with
rounded, condensed nuclei & unstained
cytoplasm
Oligodendrocytes
• Glial cells derived from
neuroepithelium
• Begins in lower brain stem, older tracts first
• 6th months through puberty
CNS: oligodendrocytes
• NCC derived
• Motor roots myelinated
before sensory roots
• 4th month
PNS: Schwann cells
• ? cells that line
the fluid-filled ventricles of the brain &
the central canal of the spinal cord
• Apical end may have cilia & long microvilli
- Facilitate movement of CSF
- Likely involved in absorption
• Joined apically by apical junctional complexes & there is no ?
Ependymal Cells
- Columnar or cuboidal
- basal lamina
• Elaborately folded & highly vascular tissue,
found in the roofs of the ?, & in parts of ?
• Contains a thin layer of well-vascularized pia
mater covered by ? ependymal cells
• Removes H2O from blood & releases it as CSF – Contains ?, ?, & ? ions – Very little protein – Only cells include sparse lymphocytes
- Completely fills ventricles, central canal of SC, subarachnoid & perivascular spaces
- ? provide absorption pathway for CSF back into the venous circulation
Choroid Plexus
- 3rd & 4th ventricles
- lateral ventricular walls
- columnar to cuboidal
- Na+, K+, & Cl-
- Arachnoid villi
• Less numerous than oligodendrocytes or
astrocytes
• Migratory, will remove damaged or unactive synapses or other fibrous
components
• Major mechanism of immune defense in
the CNS, removing any microbial invaders
• Originate from monocytes
Microglia
Cerebellar Cortex
- has much neuropil & scattered neuronal cell bodies
Molecular Layer
Cerebellar Cortex
- extend dendrites throughout the molecular layer as a branching basket of nerve fibers
- Conspicuous in H&E-stained sections
Purkinje Cells
Cerebellar Cortex
- contains various very small, densely packed neurons (i.e., granule cells)
& little neuropil
Granular layer
Cerebellar Cortex
- Organized into folia with the ? located deep
cerebellar medulla
Cerebral Cortex
- Biologically older parts of the cortex are arranged into three layers
Archicortex
Cerebral Cortex
- Most of the cortex (~90%)
- Consists of 6 layers
Neocortex
Cerebral Cortex
- pyramid-shaped cell bodies with the apex directed towards the
cortical surface
- Huge upper motor neurons of the motor cortex (Betz cells), are the largest of these cells
Pyramidal Cells
Cerebral Cortex
- small neurons with a cell body the shape of a star (look like granules in micrographs)
Granule (stellate) cells
Cerebral Cortex
- Small polygonal cells with a few short dendrites
Cells of Martinotti
Cerebral Cortex
- Spindle-shaped cells oriented at right angles to the surface of the
cerebral cortex
Fusiform cells
Cerebral Cortex
- small & spindle-shaped but oriented parallel to the surface (least common)
Horizontal cells of Cajal
Cortical Layers
- consists largely of fibers, most of which travel parallel to the surface, & mostly neuroglial cells & horizontal cells of Cajal
Plexiform Layer (or molecular layer)
Cortical Layers
- consists mainly of small pyramidal cells & granule cells
Small Pyramidal Cell Layer (or outer granular layer)
Cortical Layers
- pyramidal cells are somewhat larger and possess a typical pyramidal shape; not sharply demarcated from layer II
Layer of Medium Pyramidal Cells (or layer of outer pyramidal cells)
Cortical Layers
- is characterized by the presence of many small granule cells
Granular Layer (or inner granular layer)
Cortical Layers
- contains pyramidal cells are extremely large (Betz cells) in motor areas
Large Pyramidal Cell Layer (or inner layer of pyramidal cells)
Cortical Layers
- contains cells with diverse shapes, many are fusiform cells
Polymorphic Layer
• Function in the “limbic system“
• Integrates inputs various cortical areas into complicated behaviors
- Learning, memory, & social interaction
• Morphologically is considered ?,
- Comprised of three layers (rather the six layers found in the cerebral cortex)
The Hippocampus & Dentate Gyrus
- archicortex
Hippocampus Layer:
- nerve fibers & small cell bodies of interneurons
Polymorphic layer
Hippocampus Layer:
- hippocampal pyramidal cells
Middle pyramidal cell layer
Hippocampus Layer:
- Dendrites of the pyramidal cells
Molecular layer
Dentate gyrus Layer:
- Nerve fibers (known as “mossy fibers”) & cell bodies of interneurons
Polymorphic layer
Dentate gyrus Layer:
- Round, neuronal cell bodies of dentate granule cells
Middle granule cell layer
Dentate gyrus Layer:
- Containing dendrites of the granule cells
Molecular Layer
The region where the head of hippocampus abuts the dentate gyrus
hilus
• Most common neurodegenerative disease,
incidence increases with age
- Typical presentation is 70+ years
• Symptoms:
- Memory failure, progressing steadily to involve motor skills, speech & sensation
• Etiology is unknown, small proportion of
cases have a genetic association
• Striking thinning of gyri, particularly those
of the ? and ? lobes
Alzheimer’s disease
- frontal and temporal
Alzheimer’s disease
- amorphous, pink
masses in the cortex
Amyloid (amyloid β) plaques
Alzheimer’s disease
- flame-shaped skeins formed by abnormal accumulation of tau
Neurofibrillary tangles
